A special attention is paid to the radio LAN that frees users from the wired LAN architecture. The radio LAN can eliminate most cables in working spaces such as offices, making it possible to relatively easily move communication terminals such as personal computers (PCs). In recent years, there is an increasing demand for wireless LAN systems in accordance with their increased processing speeds and reduced prices. Today, especially, introduction of a personal area network (PAN) is taken into consideration for the sake of information communication by constituting a small-scale wireless network between a plurality of electronic devices around a person. For example, there are provided different wireless communication systems using frequency bands such as 2.4 GHz and 5 GHz bands that need not be licensed by governing legal authorities.
The radio network performance is dramatically increased while the integration and energy saving are improved for LSI technologies. Radio networks are used worldwide and are being standardized. Prices of radio LAN apparatuses have been reduced approximately as low as computer peripherals. The use of radio networks is not limited to conventional computer networks, but is diversely intended for connection with peripherals in an office, transmission of high-quality video such as streams between home information appliances in a house, and the like.
Typical standards concerning radio networks include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., see non-patent document 1), HiperLAN/2 (e.g., see non-patent document 2 or 3), IEEE802.15.3, and Bluetooth communication. The IEEE802.11 standard is further classified into various radio communication systems such as IEEE802.11a, IEEE802.11b, and the like depending on radio communication systems and frequency bands to be used.
In recent years, special attention is paid to the “Ultra-Wideband (UWB) communication,” i.e., as a radio communication system to realize the short-range ultra-fast transmission and is expected to be in practical use (e.g., see non-patent document 4). The UWB communication performs radio communication by using ultra-short pulse shorter than one nanosecond to carry information without using carriers in a very wide frequency band. Presently, for example, IEEE802.15.3 is under examination of the system for transmitting packet-structured data containing preambles as an access control system for the ultra-wideband communication.
In the future, WPAN (Wireless Personal Access Network) such as UWB for short-range communication is supposed to be incorporated into all home appliances and CE (Consumer Electronics) devices. It is expected to realize P-to-P transmission between CE devices or home networks at speeds over 100 Mbps. When millimeter wave bands are widely used, it is possible to provide short-range radio communications at speeds over 1 Gbps. It is also possible to realize ultra-fast short-range DAN (Device Area Network) including storage devices and the like.
When radio networks are constructed under a working environment where there is a mixture of many devices in a room, multiple networks may be constructed to overlap with each other. The radio network using one channel has no corrective measures when another system interrupts the communication or an interference degrades the communication quality.
To solve this problem, the multi-channel communication system is used to previously provide multiple communication channels. An interference may degrade the communication quality when another system interrupts the communication or the number of participating stations increases to leave no allowance in the band. In such case, it is possible to select a communication channel to start operations, maintain network operations, and realize coexistence with the other networks.
For example, the multi-channel communication system is also used in the high-speed radio PAN system in compliance with IEEE802.15.3. That is, there are provided multiple frequency channels available for the system. According to the algorithm used, a radio communication device, when turned on, scans for all available channels. The radio communication device becomes a piconet coordinator (PNC) and confirms whether or not there is a device transmitting a beacon signal in the vicinity. The radio communication device selects a frequency channel to be used.
Constructing a radio network in a room forms the multi-path environment where a receiver receives a layer of a direct wave and multiple reflected and delayed waves. The multi-path environment generates a delay distortion (or frequency selective fading) to cause a communication error. Further, an inter-symbol interference results from the delay distortion.
Major countermeasures against the delay distortion may include the multi-carrier transmission system. The multi-carrier transmission system transmits transmission data by distributing it to multiple carriers having different frequencies. Each carrier is provided with a narrow band, making it difficult to be subject to effects of the frequency selective fading.
For example, the OFDM (Orthogonal Frequency Division Multiplexing) system, one of multi-carrier transmission systems, configures a frequency of each carrier so that the carriers become orthogonal to each other in a symbol region. During information transmission, the system converts serially transmitted information into parallel information at a symbol frequency lower than the information transmission rate. The system allocates a plurality of pieces of output data to each carrier, modulates the amplitude and the phase for each carrier, and performs the inverse FFT for the carriers. In this manner, the system converts the carriers into signals along the time axis by maintaining the orthogonality of each carrier along the frequency axis. The reception occurs in the reverse order of the transmission. The system performs the FFT to convert signals along the time axis into those along the frequency axis and demodulates the carriers in accordance with the modulation of each carrier. The system performs parallel-serial conversion to reproduce the information that was originally transmitted in the serial signals.
The OFDM modulation system is adopted as a wireless LAN standard in the IEEE802.11a/g, for example. The IEEE802.15.3 standardization is also in progress for the UWB communication system using the OFDM modulation system in addition to the DS-UWB system and the impulse-UWB system. The DS-UWB system increases spread speeds of DS information signals to the utmost limit. The impulse-UWB system uses impulse signal sequences having very short frequencies of several hundred picoseconds to configure information signals for transmission and reception. With respect to OFDM_UWB communication systems, the multiband OFDM_UWB communication system is under discussion (e.g., see non-patent document 5). The multiband OFDM_UWB communication system is an OFDM modulation system that performs frequency hopping (FH) for frequency bands ranging from 3.1 to 4.8 GHz into multiple frequency channels (sub-bands) each composed of 528 MHz bandwidths and uses IFFT/FFT with each frequency band composed of 128 points.
FIG. 17 shows a frequency allocation defined in the multiband OFDM_UWB communication system. The frequencies are composed of groups 1, 2, 3, D, and 5. Group 1 is composed of bands #1 through #3 whose center frequencies are 3432 MHz, 3960 MHz, and 4488 MHz, respectively. Group 2 is composed of bands #4 through #6 whose center frequencies are 5016 MHz, 5544 MHz, and 6072 MHz, respectively. Group 3 is composed of bands #7 through #9 whose center frequencies are 6600 MHz, 7128 MHz, and 7656 MHz, respectively. Group D is composed of bands #10 through #12 whose center frequencies are 8184 MHz, 8712 MHz, and 9240 MHz, respectively. Group 5 is composed of bands #13 and #14 whose center frequencies are 9768 MHz and 10296 MHz, respectively. It is mandatory to use three bands in group 1. The other groups and bands are reserved for the future expansion.
FIG. 18 shows an configuration example of the receiver used for the multiband OFDM system (e.g., see non-patent document 6). The receiver in FIG. 18 is configured for direct conversion. The direct conversion system removes an intermediate frequency (IF) stage. A low-noise amplifier (LNA) amplifies a signal received at an antenna. A mixer then multiplies the signal by a local frequency to directly apply the frequency conversion to a baseband signal. The example in FIG. 18 uses local (LO) signals cos(2πfc) and sin(2πfc) for frequency conversion of reception signals corresponding to the I and Q axes, respectively. After the frequency conversion, a low-pass filter (LPF) extracts low frequencies. A VGA (Variable Gain Amplifier) amplifies the signal. The AD conversion is performed. Further, the FFT is performed to transform time-axis signals to frequency-axis signals. Each carrier is demodulated to reproduce the information transmitted by the original carrier signal.
When using the group-1 band as shown in FIG. 17, for example, the direct conversion receiver as shown in FIG. 18 requires local signals having three frequencies 3432 MHz, 3960 MHz, and 4488 MHz that are the same as the RF signal's center frequencies.
The direct conversion system eliminates the use of an IF filter, easily broadens the receiver's band, and increases flexibility of the receiver configuration. However, the direct conversion system equalizes the reception frequency with the local frequency. There is a known problem that the local signal's self mixing (LO self mixing) causes a DC component, i.e., a DC offset (e.g., see non-patent document 7).
FIG. 19 shows how the local signal's self mixing occurs. The local signal leaks from the receiver body to the antenna. This signal partially reflects at the antenna and returns to the receiver. The mixer multiplies the signal and the local signal itself together. In another possible case, the local signal is partially radiated to the outside through the antenna. Subsequently, a reflected wave may be received at the antenna and is mixed with the local signal.
In FIG. 19, for example, let us assume that the local signal's amplitude is 0.5 V; the low-noise amplifier (LNA) and the mixer provide a total gain of 30 dB; and a leaking local signal reflects at the antenna and returns to point A in FIG. 19 to attenuate −70 dB. Under this condition, the DC offset for mixer output is found to be 2.5 mV. Since the expected wave's signal level is approximately −74 dBm at the minimum, the mixer output becomes −44 dBm=1.4 mVrms. It can be understood that the DC offset becomes greater than the expected wave's signal level.
The following equation describes a process to generate the DC offset. In the equation, cos(ωt) represents the local signal and α and φ respectively represent the amplitude and the phase of a reflected wave returned to the mixer. In the equation, the first term of the right side represents the DC offset. The second and third terms represent double frequency components. It will be understood that the DC offset varies with the reflected wave's amplitude and phase.
                              α          ·                      cos            ⁡                          (                                                ω                  ·                  t                                +                ϕ                            )                                ·                      cos            ⁡                          (                              ω                ·                t                            )                                      =                              1            2                    ·          α          ·                      (                                          cos                ⁢                                  (                  ϕ                  )                                            +                                                                    cos                    ⁡                                          (                      ϕ                      )                                                        ·                  cos                                ⁢                                  (                                      2                    ·                    ω                    ·                    t                                    )                                            -                                                sin                  ⁡                                      (                    ϕ                    )                                                  ·                                  sin                  ⁡                                      (                                          2                      ·                      ω                      ·                      t                                        )                                                                        )                                              [                  Equation          ⁢                                          ⁢          1                ]            
Since the multiband OFDM system performs the frequency hopping (FH), the local signal frequency varies with each frequency hopping. Since the antenna's reflection coefficient depends on frequencies, the DC offset resulting from self-mixing varies with the frequency hopping. The frequency hopping occurs as frequently as 3.2 MHz equal to the OFDM symbol rate. As shown in FIG. 20, the DC offset stepwise varies at the frequency of 1/3.2 MHz=312.5 nanoseconds.
Generally, the DC offset is removed by serially inserting a capacitor in the mixer output. In this case, capacitor C and circuit impedance R construct a first-order high-pass filter (HPF). The cutoff frequency for frequency response becomes 1/(2πCR). The convergence time for step response becomes 2πCR.
Since the multiband OFDM system uses the sub-carrier frequency of 4.125 MHz, the direct conversion receiver is requested to pass up to 4.125 MHz. With respect to the DC offset, the convergence time for step response needs to be limited to approximately 1/10 of the OFDM symbol rate (approximately 30 nanoseconds). When the cutoff frequency is set to 4.125 MHz, the time to converge the step response becomes as large as 242 nanoseconds (=1/4.125 MHz) as shown in FIG. 22. The problem is that the most time within the OFDM symbol is accompanied by the step response.
Generally, available means of changing frequencies is to multiply the same oscillatory frequency using PLL (Phase Lock Loop). However, there is the problem that the multiband OFDM_UWB system is subject to a large difference in the channel changeover as shown in FIG. 17. The single PLL cannot change frequencies within such wide band. A high-precision multiband generator can be constructed by providing multiple oscillators and generating corresponding frequency bands. However, there may be problems concerning the circuit's planar dimension and power consumption, a frequency phase difference for each oscillator, and the like.
To solve this problem, multiband generation is performed by repeatedly dividing the single frequency output from the oscillator and mixing divided output frequencies (i.e., outputting a sum or a difference between frequencies).
FIG. 23 diagramatically shows a conventional example of the frequency synthesis block (group-1 3-band mode) for frequency hopping (FH) used for the direct conversion receiver as shown in FIG. 18 of the multiband OFDM system. As shown in FIG. 23, the divider and the mixer can be used to mix (add or subtract frequencies) the center frequency of each band with the reference frequency obtained from the single oscillator (e.g., TCXO (Temperature Compensated Crystal Oscillator)).
The example in FIG. 23 uses the reference frequency of 4224 MHz obtained by multiplying the oscillatory frequency output from the oscillator according to PLL (Phase Lock Loop). The reference frequency is divided by 4 to extract the frequency of 1056 MHz. This is divided by 2 to extract the frequency of 528 MHz that is used for a sample clock. The 528 MHz sample clock is further divided by 2 to extract the frequency of 264 MHz that is a band interval of center frequencies for frequency hopping.
Each mixer marked with SSB (Single Side Band) performs mixing, i.e., adding or subtracting frequencies of the obtained signals as mentioned above. The mixer adds frequencies of 528 MHz and 264 MHz to yield the frequency of 794 MHz. A selector (Select) selects 264 MHz or 794 MHz. The subsequent SSB can provide four combinations of frequencies by performing addition or subtraction between the selected output frequency 264 MHz or 794 MHz and the original 4224 MHz frequency signal.
It should be noted that group 1 uses only three frequencies 3432 MHz, 3960 MHz, and 4488 MHz. That is, the 792 MHz frequency is subtracted from 4224 MHz to generate 3422 MHz. The 264 MHz frequency is subtracted from 4224 MHz to generate 3960 MHz. The 264 MHz frequency is added to 4224 MHz to generate 4488 MHz.
In FIG. 23, the device marked with SSB mixes, i.e., adds or subtracts frequencies and is equivalent to an image rejection mixer, for example. The image rejection mixer can obtain a single-side-band signal by analog-multiplying a pair of complex signals whose phases are orthogonal to each other. As shown in FIG. 24, each of frequency signals f1 and f2 is provided with orthogonal components. Frequencies can be synthesized by adding or subtracting frequencies using the addition theorem of the trigonometric function. In FIG. 24, frequency signal f1 is set to 4224 MHz and frequency signal f2 is set to 264 MHz or 794 MHz.
However, the conventional frequency synthesis block as shown in FIG. 24 is subject to the following problems.
(1) Since two SSB mixers are needed, the circuit configuration is complicated and the power consumption is large.
(2) Since the 264 MHz frequency signal is a rectangular wave, third harmonics cause a spurious signal of approximately up to −10 dBc in group 1.
Specifically, the preceding SSB for 792 MHz generation is supplied with not only 528 MHz and 264 MHz, but also the third harmonic of 264 MHz, i.e., −792 HMz. The SSB outputs not only 792 MHz as an intended frequency, but also −264 MHz, causing a spurious signal in group 1.
(3) Since the 264 MHz frequency signal is a rectangular wave, fifth harmonics cause a spurious signal of approximately up to −14 dBc in group 1.
[Non-Patent Document 1]
International Standard ISO/IEC 8802-11: 1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-Patent Document 2]
ETSI Standard ETSI TS101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part1: Basic Data Transport Functions
[Non-Patent Document 3]
ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2: Radio Link Control(RLC) sublayer
[Non-Patent Document 4]
“Ultra Wideband—A newborn revolutionary wireless technology.” Nikkei Electronics, 11 Mar. 2002, pp. 55-66.
[Non-Patent Document 5]
IEEE802.15.3a TI Document <URL:http://grouper.ieee.org/group/802/15/pub/2003/May03 filename: 03142r2P802-15_TI-CFP-Document.doc>
[Non-Patent Document 6]
Anuj Batra, “03267r1P802-15_TG3a-Multi-band-OFDM-CFP-Presentation.ppt,” pp. 17, July 2003.
[Non-Patent Document 7]
Asad A. Abidi, “Direct-Conversion Radio Transceivers for Digital Communications” (IEEE J. Solid-State Circuits, vol. 30, no. 12, pp. 1399-1410, 1995